You can make terra sig from almost any clay, but some work better than others. Its going to depend on what clay you are throwing with.

Porcelain makes lousy sig, but I use whatever else wanders through the studio. We have basic white stoneware sig for adding mason stain to, sigs from several red clays, sig from black clay, sig from clay from my flower bed, you get the picture. The sig often fires a different color than the original clay because of the way the particles separate. My favorite and best burnishing are the flowerbed clay and the black (it fires chocolate brown.) You're just going to have to give it a whirl and see what you get. The worst part of using your slops is going to be figuring out how much deflocculant to add if you are a recipe person. I usually wing it with good luck, but I've had some practice.